The recirculation of some engine exhaust gas is a recognized technique for achieving compliance with applicable tailpipe emission requirements. NOX emissions in particular. Various EGR strategies have been proposed for turbocharged engines, including diesel engines. Examples appear in U.S. Pat. Nos. 6,422,217; 6,263,672; 6,000,222; 5771,867; 5,671,600; and 4,215,550.
U.S. Pat. No. 6,422,217 describes a device that is placed in an engine exhaust system between the engine exhaust manifold and the turbocharger turbine. The device contains a valve mechanism for controlling engine back-pressure to induce recirculation flow through an EGR flow path to the engine intake system. The EGR flow path contains an EGR cooler. The cooled FOR flow enters a mixer where it entrains with charge air from the turbocharger compressor.
U.S. Pat. No. 6,263,672 describes a similar system having separate back-pressure and EGR valves instead of a single device.
U.S. Pat. No. 6,000,222 describes a turbocharger that integrates an EGR valve with the turbine housing. A back-pressure valve is fastened to the turbine housing. Although the patent contains no schematic diagram of EGR flow, it is believed that the reader is led to understand that the pierce point to the intake system is downstream of the turbocharger compressor.
U.S. Pat. No. 5,771,867 describes another system where the pierce point to the intake system is downstream of the turbocharger compressor.
U.S. Pat. No. 5,671,600 describes a system where the EGR pierce point to the exhaust system is upstream of the turbocharger turbine. The EGR flow path contains a particulate filter upstream of an EGR valve. The pierce point to the intake system is upstream of the turbocharger compressor.
U.S. Pat. No. 4,215,550 describes several systems some of which have the EGR pierce point to the exhaust system upstream of the turbocharger turbine and others of which have it downstream. The pierce point to the intake system is downstream of the turbocharger compressor.
An EGR system that has the pierce point to the exhaust system downstream of the turbocharger turbine may be considered a Low-Pressure EGR (LP EGR) system. Such a system possesses certain advantages. The extent to which such a system can provide increased EGR flow that is desired in order to eliminate more NOx from tailpipe emissions is limited by compressor inlet depression.
Throttling of engine intake flow is a known technique for achieving compressor inlet depression conducive to use of LP EGR.